Non-photosynthetic Corals: They really are hard!

In the last 15 years, the ability to recreate a piece of a
living reef in ones home has reached unprecedented heights. With
the development of high intensity lighting, the maintenance of
corals that require light has become possible and is now within
the reach of almost any aquarist. Corals such as Acropora,Montipora, and Seriatopora are now commonly grown
and reproduced through fragmentation in home aquariums. However,
there are still a group of corals that, despite all the advances
of the past fifteen years are still proving almost impossible to
keep. Although there are isolated cases where some success has
been reported, the maintenance of these corals for long periods
of time, where growth and propagation have approached natural
rates, still remains elusive. Commonly know as azooxanthellate or
non-photosynthetic corals, they include several families and
genera. These include soft corals such as Chironephthya,Dendronephthya, Scleronephthya,Siphonogorgia and Stereonephthya, gorgonians such
as Acabaria, Acalcygorgia, Melithaea andSubergorgia, black corals and wire corals such asAntipathes and Cirripathes, hydrocorals such asStylaster and Distichopora, and of course stony
corals in the genus Tubastraea. Although the term “soft”
coral is used to describe a grouping of corals, it should be
noted that this is a generic term used primarily for corals in
the suborder Alcyoniia.

Picture of stream tank. Photo: S
Brown.

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Schematic diagram of a simple flow tank using a motor driven propeller.

Stream tank using motor driven propeller.

The reasons for the poor success rates with the majority of
these corals can be traced to fact that they need to feed on
plankton. Enough food, of the right type and size must be
provided. Until recently, very little was known about the feeding
requirements of these corals. The vast majority of soft corals
and gorgonians available in the hobby rely greatly on
zooxanthellae for their nutrition. However, recent studies have
shown zooxanthellae may not be able to meet the total nutritional
needs of all soft corals. Fabricius and Klumpp (1995) found that
twelve of the most common photosynthetic soft coral species
investigated on the Great Barrier Reef could not meet their
carbon requirements by photosynthesis alone. This brings up the
question of just where do they get their carbon? Many octocorals
are known as polytrophic feeders, meaning that they are capable
of obtaining nutrition from more than one source (Williams,
1993). Possible sources may be one or all of the following: the
direct absorption of nutrients, the ingestion of zooplankton
and/or phytoplankton, the ingestion of “marine snow” along with
its attached bacteria and organic material. Several studies have
shown that soft corals, gorgonians and sea pens can feed on a
variety of zooplankton such as copepod nauplii and eggs,
invertebrate eggs and other small items of poor mobility. Many of
these studies, however, were conducted in the laboratory, using
artificial foods (Artemia) or concentrated natural
zooplankton of unknown density (Fabricius et al., 1995a).
These studies showed that octocorals tend to be highly selective
for non-evasive forms such as mollusc larvae; indicating poor
capture ability of more elusive prey such as large adult
copepods. This poor capture ability is most likely due to the
lack of effective nematocysts, resulting in the selection of less
motile prey (Fabricius et al., 1995a). In fact, Fabricius
(unpublished data) found that an inability to feed on zooplankton
was widespread amongst zooxanthellate soft coral genera on the
Great Barrier Reef (i.e. three species of Sarcophyton, two
species of Sinularia, Cladiella sp.,Nephthea sp. and Paralemnalia sp.). The role that
zooplankton play in the nutrition of photosynthetic octocorals
is, as yet, unclear but new information is showing that they
contribute only a small portion to the nutritional budget of many
octocorals (Fabricius et al., 1995a and b). However, many
of the studies that looked at a corals ability to feed on
zooplankton often used Artemia nauplii as prey items under
controlled situations. Artemia are rather large, and it
may not be surprising given the small size and weak nematocysts
of many soft corals, that they are not easily captured. Perhaps
smaller zooplankton such as copepod nauplii, rotifers, or marine
infusoria is fed upon? However, the question remains, if not
zooplankton, then what are their main prey items?

Phytoplankton is an order of magnitude more common on coral
reefs than zooplankton. Studies have shown that phytoplankton is
somehow depleted over corals reefs, though where it goes no one
knows (in Fabricius et al., 1995a). In 1961, Roushdy and
Hansen showed that the asymbiotic soft coral Alcyonium
digitatum feed on 14C labeled
phytoplankton (in Fabricius et al., 1995b). In 1969, it
was demonstrated that the temperate water sea pen Ptilosarcus
gurneyi fed primarily on phytoplankton; its bright orange
colour, the result of carotenoids derived from a diet of
dinoflagellates (in Best, 1988). Elyakova et al. (1981),
in a general survey of carbohydrases in marine invertebrates,
found the presence of laminarinase and amylase in three species
of the zooxanthellate soft coral genus Alcyonium, enzymes
involved in the digestion of plant material. It was not until
1995 that Fabricius et al. published papers that
demonstrated quite clearly that the Red Sea azooxanthellate soft
coral Dendronephthya hemprichi, fed extensively on
phytoplankton, gaining more than enough carbon to cover
respiration and growth requirements. Although this species also
fed on zooplankton, only 2.4-3.5% of the daily carbon requirement
of this coral was met by ingesting zooplankton. Three other
asymbiotic Red Sea octocorals, D. sinaiensis,Scleronephthya corymbosa and the gorgonianAcabaria, were also found to contain large quantities of
phytoplankton in their gastrovascular cavities (Fabricius et
al., 1995b). Adaptations for phytoplankton capture include
the small spaces between the pinnules of D. hemprichi,
which appear to be ideal for straining phytoplankton from flowing
waters. The large spicules found in the body column and around
the polyps of Dendronephthya spp., appear to function more
in holding the column and polyps erect in strong current flows,
than to protect against predation, allowing the polyps to strain
phytoplankton effectively from the passing waters (Fabriciuset al., 1995a). Some of the most impressive
growths of Dendronephthya spp. are often found on
shipwrecks in the South Pacific, where structures high above the
bottom and projecting into the current are often heavily
encrusted. It is tempting to equate this with oyster hatcheries,
where oysters are hung in cages well above the bottom and within
strong currents. Both organisms feed on phytoplankton, and hence
benefit from these positions by being exposed to maximal
phytoplankton concentrations. In light of this new evidence,
scientists need to re-evaluate the role of phytoplankton in the
nutrition of other octocorals. Several studies are now underway
to determine to what extent both zooxanthellate and
azooxanthellate species actually feed on phytoplankton.

Another mode of feeding may be the trapping
of mucus flocs often called marine “snow”. These are composed of detritus,
bacteria, protozoans and possibly phytoplankton trapped in mucus. The source
of this mucus is most likely soft and stony corals, which rid themselves
of epizoic growths and excess carbon and fats, by releasing mucus. This
mucus is not easily degraded by bacteria and is often infested with large
quantities of bacteria and eukaryotes (flagellates, ciliates and diatoms)
(Vacelet and Thomassin, 1991). These mucus flocs could be trapped by the
spiky polyps of Dendronephthya spp. and used as a food source. It
is fully possible that octocorals employ a combination of some or all of
the above feeding mechanisms, with varying degrees of importance for each.

Cylinder tank used for Dendronephthya research. Note central stand pipe
and twin return pipes at the back of the tank each connected to a separate
pump and timer. Photo: Norton Chan.

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Armed with the above information, first
presented to hobbyists in The Reef Aquarium volume two, (Sprung and
Delbeek, 1997) several North American aquarium supply companies now make
available various mixtures of phytoplankton, some live, some cryo-preserved,
and some consisting of dead algal cells. Most of these products were originally
developed for the aquaculture industry. Although phytoplankton may be of
importance to soft corals, its direct role for other non-photosynthetic
corals is not as well demonstrated, and may indeed be questionable. Certainly
corals such as Tubstraea spp. and antipatharians are known to prey
heavily on zooplankton. The needs of some gorgonians can also be well-met
using zooplankton substitutes such as enriched Artemia, rotifers
and copepods.

Tank soon after filling and the addition of corals. Both photos: Norton
Chan

As it turns out, the foods required for success are
only one piece of the puzzle. Another factor, equally important
is water motion; not only the type but also the velocity of water
motion can be critical for some genera, but less critical for
others. Given that octocoral polyps have few, small stinging
cells (or none at all) and that their pinnules offer a large
surface area, they are generally classified as suspension
feeders, straining fine particles from the passing water. As such
their feeding efficiency is affected by the rate of current flow,
polyp and colony flexibility, and orientation. Several studies
have shown that feeding efficiency generally increases up to a
maximum velocity and then drops off at velocities beyond that
(Best, 1988; Sponaugle and LaBarbera, 1991; Dai and Lin, 1993;
Fabricius, et al., 1995a). However, the flexion of the
polyps and colony can act together to increase the range of
current velocities over which suspension feeding is successful
(Sponaugle, 1991).

The polyps themselves can actually modulate the flow
around them, to enhance prey capture. In a study of the effects
of flow on particle capture in the asymbiotic temperate octocoralAlcyonium siderium, Patterson (1991) found that at low
flows (2.7 cm/s) tentacles on the upstream side of the polyps
capture the most prey. At intermediate flows (12.2 cm/s)
downstream tentacles within a polyp capture the most prey. In
high flow (19.8 cm/s) polyps are bent downstream, eddies form
over the polyp surfaces and all tentacles capture prey
effectively. Prey is trapped most effectively at the tips of the
tentacles relative to locations near the mouth (Patterson, 1991).
No one current flow is the best for all species. For example, Dai
and Lin (1993) found three Taiwanese asymbiotic gorgoniansSubergorgia suberosa, Acanthogorgia vegae andMelithaea ochracea to feed over a wide range of flow
rates. The ability to keep polyps open was also related to flow
rates and the size of their polyps. Subergorgia suberosa
had the largest polyps, which were deformed by the lowest
currents speeds (>10 cm/s), severely hindering prey capture.
In contrast, Melithaea ochracea, which had the shortest
and the least easily deformed polyps at high flow rates, could
feed at the highest flow rates (40 cm/s). Acanthogorgia
vegae had an intermediate polyp size and fed in flows of 0-24
cm/s. Although all three fed most effectively at flows of 8 cm/s,S. suberosa had the narrowest feeding range (5-10 cm/s)
while M. ochracea had the widest range (4-40 cm/s) (Dai
and Lin, 1993). This varying ability to feed in various current
flows is a major factor in determining distribution on reefs.Melithaea ochracea is the most widely spread gorgonian on
southern Taiwanese reefs, occurring on the upper part of reef
fronts where currents are strong. Subergorgia suberosa,
which feeds in a narrow range of flow velocities, has a
restricted distribution pattern, being found on lower reef slopes
or on sheltered boulders where currents are weaker.Acanthogorgia vegae, which can feed in relatively strong
currents, is most commonly found on the semi-exposed reef fronts
or the lateral side of boulders (Dai and Lin,
1993).

Therefore, water flow and its interactions with
polyps and colonies, appears to greatly influence distribution
patterns of colonies, colony growth, size and morphology, and
rates of gas exchange (in Fabricus et al., 1995a). To
summarize, in increasing flows, feeding rates initially increase,
peak, then decrease as flow rate increases. Having too great a
flow can also cause polyps to stay open for shorter and shorter
periods of time and having flow rates too low do not stimulate
polyps to open and feed.

Tank with various corals, one month after collection. Photo: Norton Chan.

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In aquaria, water motion is predominantly of two
types, laminar and chaotic. Laminar flows occur close to the
outlets of powerheads and water returns. Chaotic flows begin to
appear further from these sources as the flowing water encounters
resistance from water, tank walls, rocks and corals. Areas where
two water flows intersect also provide areas of chaotic water
motion. However, the areas where the majority of
non-photosynthetic corals appear on reefs, (along drop-offs and
channels in reefs) have laminar flows that usually operate on
tidal cycles. Water flow will gradually increase in one direction
then decrease, then change direction and increase again. There
are periods of “slack” tides between these two extremes. The
corals in these regions therefore receive strong laminar flow in
one direction for several hours followed by a slack period, then
another period of strong water flow in the opposite direction for
several hours. To stimulate this in aquaria is difficult and may
require the design of new tank designs and water motion
devices.

At the Waikiki Aquarium we have been working with a
cylindrical tank to simulate these conditions with two water
pumps operated by timers. The tank contains live sand, some live
rock and has a continuous flow of natural seawater, and so no
other filtration is required. Lighting is supplied by ambient
natural sunlight coming from overhead acrylic panels with some
direct sunlight for a few hours each day. Other tank designs such
as flumes and flow tanks could also be used. For example, at the
Vancouver Aquarium, in Canada, an exhibit of coldwater gorgonians
has been constructed that uses a flow tank with high horsepower
pumps to simulate the very strong tidal currents that occur in
some of the passes between the islands offshore of British
Columbia.

Dendronephthya along outer edge. Photo: Norton Chan.

At the Waikiki Aquarium we have had some moderate
success with keeping certain species of black coral, and good
success with wire corals by feeding them a diet of enrichedArtemia and copepods. The Long Beach Aquarium of the
Pacific in California has had some success withDendronephthya,Distichopora and some other soft
corals using a diet of phytoplankton including Chlorella
sp., Spirulina sp., Isochrysis sp., andNanochloropsis sp. To that algae “soup”, they add rotifers
and supplemented Artemia. They also found thatDistichopora, unlike non-photosynthetic soft corals,
unquestionably require low light levels. If left in even moderate
light, fouling organisms quickly adhere to their delicate tissues
and result in mortality. The Shedd Aquarium in Chicago, has had
one colony of Dendronephthya for over a year, which has
shown noticeable growth. They are feeding phytoplankton as well
as live rotifers and copepods. Our success withDendronephthya has been less inspiring.

In early December 2000, we
collected fifteen small colonies of Dendronephthya in Fiji
and transported them back to Hawaii under permit. At present
(March 17, 2001) we have seven of the fifteen colonies still
surviving. Although we have tried several food types such as live
marine phytoplankton (Chaetoceros,Isochrysis,
Nannochloropsis etc.), copepods, rotifers, fatty acid
supplements and “marine snow” products, we have had mixed
results. In some cases, damaged colonies quickly regrow polyps
and attain new tissue, however, established colonies slowly
decreased in size. Interestingly enough, the greatest reaction to
substances added to the aquarium occurs in two ways. When the
interior of the glass is wiped of algae the colonies show an
increase in size only an hour or so later. Secondly, when the
juice from thawed frozen squid is added to the aquarium, colonies
show the greatest increase in expansion. It is likely that these
corals feed to a greater extent on zooplankton then current
research has indicated and aquarists should not rely solely on
phytoplankton as a food source. Peter Wilkens has been able to
keep small colonies for some time in his aquaria by occasionally
stirring the bottom substratum, releasing detritus and quite
possibly bacteria and other infauna, on which the corals may
feed.

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In April of 2001, our
director Dr. Bruce Carlson returned from Fiji with a specimen of
the gorgonian Menella. With its magenta tissue and
snow-white polyps it is a beautiful sight when fully opened! As
with the Dendronephthya we had collected previously, this
coral opens its polyps most frequently when squid juice is added
or the tank window is cleaned. While closely observing the
polyps, live Artemia naupilli, live rotifers and copepods
were added in separate feeding trials. Even polyps that hadArtemia directly applied to them could not hold onto the
struggling live food, and were eventually released. Next, live
microalgae cultures were tried as well as Algamac, a commercially
available artificial phytoplankton substance (see
www.argent-labs.com). During these trials individual arms of the
polyps could be seen periodically bending towards the mouth,
wiping along the aboral surface of the polyp. I believe that
mucus on these arms and their associated pinnules may be trapping
passing phytoplankton cells then passing them to the mouth to be
ingested.

Orientation of colonies is another factor that can
play a significant role. Colonies that were placed upright on the
sand bottom in our system initially appeared to do well, but over
time, began to shrink in size. When placed upside-down from a
supporting structure, these colonies slowly recovered and looked
much better, some even showing new growth and reattachment to the
substratum. The key seems to be to not allow the colonies to
touch the bottom with their branches; this seems to irritate them
over time and results in the loss of spicules. This is of
greatest concern when the colony is deflated. Another interesting
observation is that colony inflation and deflation does not seem
to follow any discernable pattern. In the early morning the
colonies are deflated and then inflate later in the morning and
would remain so for most of the day, although deflation could
occur again at any time during the day.

Dendronephthya sp. at 120 ft., Solomon Islands. Photo: JC Delbeek

As I mentioned briefly above, the Waikiki Aquarium
has had some success in maintaining live wire corals and black
corals. The wire coral Cirrhipathes anguina was easily
maintained on live Artemia naupilli enriched with Super Selco and
Algamac, and growth was readily visible. The black coral,Antipathes dichotoma, fed on live copepods but growth was
not as noticeable and the colonies would slowly deteriorate with
time. Recently, I tried feeding this coral frozen copepods from
Argent called Cyclopeez, which I was feeding to myPseudanthias tank at the time. These were rather large
compared to Artemia, about twice as wide. They are also
enriched with various pigments and appear bright
orange/red.

I placed a small branch in a dish of seawater and
placed this under a dissecting microscope. I used an eyedropper
to add a small amount of Cyclop-eez to the expanded polyps. To my
surprise the polyps easily grasped these large copepods and would
engulf the entire animal by expanding the mouth until the entire
animal could be passed in. Apparently the fact that these were
not moving targets helped the polyps capture and ingest them as
opposed to a struggling live prey item. I assume that the lack of
any water movement also made prey capture easier. Unfortunately,
I have not had time to pursue this due to other projects, but I
would like to attempt a long term study by feeding this food item
to a colony over a period of months and see whether or not it’s
growth and survival improves over that of copepod andArtemia fed colonies.

There are several questions that remain to be
answered in keeping non-photosynthetic corals. The role of
temperature, colony orientation, nutritional composition of foods
(including pigments), food density, the best techniques for coral
collection, and handling and shipping are just some that need to
be investigated over the next few years. We are beginning to see
limited success with many of the non-photosynthetic corals that
used to be very difficult to keep, and it is only a matter of
time until tanks filled with colorful, healthy non-photosynthetic
organisms will be as common as tanks filled withSarcophyton are today.